Matthew 27:42 He saved others; himself he cannot save. If he be the King of Israel, let him now come down from the cross, and we will believe him.
Matthew 27:43 He trusted in God; let him deliver him now, if he will have him: for he said, I am the Son of God.
Matthew 27:44 The thieves also, which were crucified with him, cast the same in his teeth.
Others were mocking by saying this but I don't think the thieves did the same thing...they "cast the same in his teeth" which I interpret to be they also wondered why Christ didn't save himself...but for other reasons than the priests were saying it. Even if they did mock the same as the others did, perhaps it was an initial reaction of anger and fear of their own deaths but after awhile I highly doubt they would be in the mood to be mocking anyone.
I think its important that there are gospels that don't mention the one who believed in Christ and what Christ said to him...I happen to believe it was still one of these thieves mentioned right here but that the conversation isnt recorded for whatever reason. Perhaps after awhile and the fate set in his mind, the one reached out to Christ and said what is recorded in the other gospel.
I also find it important that no gospel says Christ was crucified with four others...its always two others and I do not see it ever adding a second two.
John 19:18 Where they crucified him, and two other with him, on either side one, and Jesus in the midst.
Again, John records only two crucified with Christ.
John 19:32 Then came the soldiers, and brake the legs of the first, and of the other which was crucified with him.
John 19:33 But when they came to Jesus, and saw that he was dead already, they brake not his legs:
Here they come to the first, then go to the other who is on the other side and then return back to Christ lastly. Perhaps he looked deader than the others and they decided to deal with the other two first...or for some divine reason Christ was come to lastly...who knows but I don't see this as anywhere saying they approached the first two on one side and then came to Christ...its pretty clear there were only three from the beginning to the end.
Matthew 27:38 Then were there two thieves crucified with him, one on the right hand, and another on the left.
Mark 15:27 And with him they crucify two thieves; the one on his right hand, and the other on his left.
Luke 23:33 And when they were come to the place, which is called Calvary, there they crucified him, and the malefactors, one on the right hand, and the other on the left.
John 19:18 Where they crucified him, and two other with him, on either side one, and Jesus in the midst.
I see no disharmony or conflicts in the four gospels. Christ was crucified with two others, one on Christ's right hand, one on His left hand...no more or less.